1. Field of the Invention
The present invention relates to a radio communication system, and more particularly, to a random access procedure in a radio communication system.
2. Discussion of the Related Art
An enhanced universal mobile telecommunication system (E-UMTS) is an evolved version of a UMTS and basic standardization thereof is in progress under the 3rd generation partnership project (3GPP). For details of the technical specifications of the UMTS and the E-UMTS, refer to Release 7 and Release 8 of “3rd Generation Partnership Project; Technical Specification Group Radio Access Network”, respectively.
FIG. 1 is a view showing a random access procedure.
The random access procedure is used to transmit short-length data in uplink. For example, the random access procedure is performed upon initial access in an RRC idle mode, upon initial access after radio link failure, upon handover requiring the random access procedure, and upon the occurrence of uplink/downlink data requiring the random access procedure during an RRC connected mode. Some RRC messages such as an RRC connection request message, a cell update message, and an URA update message are transmitted using a random access procedure. Logical channels such as a Common Control Channel (CCCH), a Dedicated Control Channel (DCCH), or a Dedicated Traffic Channel (DTCH) can be mapped to a transport channel (RACH). The transport channel (RACH) can be mapped to a physical channel (e.g., Physical Random Access Channel (PRACH)). When a UE MAC layer instructs a UE physical layer to transmit a PRACH, the UE physical layer first selects an access slot and a signature and transmits a PRACH preamble in uplink. The random access procedure is divided into a contention-based procedure and a non-contention-based procedure.
Referring to FIG. 1, the UE receives and stores information about random access from an enode B (eNB) via system information (SI). Thereafter, if the random access is necessary, the UE transmits a random access preamble (also called a message 1) to the eNB (S110). When the eNB receives the random access preamble from the UE, the eNB transmits a random access response message (also called a message 2) to the UE (S120). In detail, downlink scheduling information for the random access response message may be CRC masked with a random access-random network temporary identifier (RA-RNTI) and may be transmitted on an L1/L2 control channel (physical downlink control channel; PDCCH). Thereafter, the UE checks whether or not random access response information associated with the UE is present in the random access response message. It may be checked whether or not the random access response information associated with the UE is present, by checking whether or not a random access preamble ID (RAID) for the preamble transmitted by the UE is present. The random access response information includes a timing advance (TA) indicating timing offset information for synchronization, radio resource allocation information used in uplink, a temporary identifier (e.g., T-CRNTI) for identifying the UE, and so on. When the UE receives the random access response information, the UE transmits an uplink message (also called a message 3) via an uplink shared channel (SCH) according to the radio resource allocation information included in the response information (S130). The eNB transmits a contention resolution message (also called a message 4) to the UE (S140) after receiving the uplink message from the UE.
If the random access of the UE fails in the above procedure, the UE may apply back-off. The back-off indicates that the UE delays the access for a certain time or a specific time. If the access is performed immediately after the random access fails, a probability that the random access fails again due to an identical or similar cause is high. Accordingly, when the random access fails, the access is delayed for a predetermined time. Thus, it is possible to prevent the waste of a radio resource due to failure or increase a probability that the random access is successfully performed.
FIG. 2 is a view showing a method of performing random access after applying back-off.
Referring to FIG. 2, a UE transmits a random access preamble using a parameter for random access in uplink. The parameter for the random access may be updated according to the number of continuous failures of the random access. The parameter includes transmission power (Tx power) for transmitting the random access preamble. For convenience, the Tx power for the transmitting the preamble is used as a representative example of the parameter. In this case, the UE uses initial Tx power with respect to the random access preamble which is initially transmitted (S210).
Thereafter, the UE receives a random access response message for the random access preamble from an eNB (S220). The random access response message may include a back-off parameter value for applying back-off. In this case, the UE stores the back-off parameter value obtained from the random access response message. Thereafter, if the random access procedure fails due to a certain cause, the UE may apply the back-off according to the back-off parameter value (S230). Since the random access procedure fails, the UE updates the parameter value for the random access and then retransmits the random access preamble. In this case, the UE retransmits the random access preamble using the Tx power increased by a ramping step (S240).
Although the random access procedure fails after the UE receives the random access response message in FIG. 2, this is only exemplary and the random access procedure may fail due to various causes. The detailed example thereof will be described with reference to FIG. 3.
FIG. 3 is a flowchart illustrating a random access procedure.
Referring to FIG. 3, a UE receives RACH information (S302). The UE performs the random access procedure as follows if a separate uplink transport channel is not present (S304).
The UE initializes a random access preamble counter M to 1 (S306). Thereafter, the UE determines whether or not the random access procedure is performed (S308). If it is determined that the random access procedure is continuously performed, then it is checked whether the value of M is equal to or less than an allowed maximum value Mmax (S310). If the value of M exceeds the allowed maximum value, the UE finishes the random access procedure (S332). If not so, the UE continuously performs the random access procedure. The UE updates the RACH information if necessary (S312), and selects a preamble signature/resource (S314). Thereafter, the UE sets the Tx power of the preamble (S316) and transmits a PRACH preamble to an eNB (S318). Although not shown in the drawing, the ramping of the Tx power of the preamble may be performed according to the value of M. That is, the Tx power of the preamble transmitted at a specific time point may be “initial Tx power+ramping step×(M−1)”.
After the preamble is transmitted, the UE checks whether or not the initial access procedure using the preamble is successful while an L1/L2 control region is monitored, in order to receive a random access response message during a specific time (S320). When the UE does not receive a downlink scheduling signal masked with an RA-RNTI during the specific time, the initial access procedure fails. In addition, although the downlink scheduling signal masked with the RA-RNTI is received and the random access response message is decoded, if the contents associated with the UE are not included in the random access response message, the initial access procedure fails. The UE increases M by 1 and may apply back-off ({circle around (1)}, S330).
When the random access response message is successfully received, the UE adjusts a transmission timing using the information obtained from the random access response and transmits an uplink message on an uplink SCH (S322). If the uplink message is not normally received by the eNB, the UE increases M by 1 (S328) and may apply the back-off ({circle around (2)}, S330).
If the uplink message is normally transmitted to the eNB, the eNB transmits a contention resolution message to the UE in downlink. Thereafter, if the contention resolution procedure is passed, the UE finishes the random access procedure. In contrast, if the contention resolution procedure is not passed, the UE increases M by 1 (S328) and may apply the back-off if necessary ({circle around (3)}, S330).
Thereafter, the UE performs the random access procedure again from the step S308.
In the related art, if the random access procedure fails, the UE retransmits the preamble with the Tx power ramped from the existing preamble Tx power after selectively applying the back-off. However, if the length of the back-off is long, a wireless environment may be changed. In such a case, if the power ramping is performed from the existing preamble Tx power, the Tx power may be improper. Accordingly, the Tx power of the UE may not be efficiently managed.